I gave up on the first of this series because it was too serious and that
was what put me off SM the most ... did it get much lighter by some of
the later episodes?

I think so... but then again. I think the anime is more darker than many
people believe it is, take SMS eps 123-125 and Stars 167-172 for example
and there's of course the cut up death eps which make a huge impression
on the seriousness of the series.  I believe SMZ contains a fairly good
mix of both elements.

Then again, there's the Brady's at Minako's school that cracked me up...
:)

Oren